Doctors and other providers working with transgender patients typically "have some sort of personal connection, whether through their politics, or they were in the LGBT community already," says Spack's former student Erickson-Schroth. Spack, on the other hand, "came to this by accident. He just saw there was something wrong in the world, and he did something."

Spack grew up in a conservative Jewish community in Coolidge Corner, where his father ran a Hebrew school and was devoted to their local synagogue. Although as a kid Spack looked up to his dad's best friend, their family physician, he never seriously considered medicine as a career until his senior year of college.

It was an accident, really, dumb luck that led him to a senior biology thesis that would set the course for the rest of his career. A student before him had been working with newts and other amphibians, and an ecology professor invited Spack to continue the research the other student had started. And so Spack sat on that research lab bench, watching the tails of these aquatic animals shrink and their legs grow for their new life on land.

The parallel isn't lost on him: he chuckles now to think of himself as a college student, using many of the same hormones he uses now, to much the same effect.

Spack went on to medical school, and then post-graduate training in adolescent medicine and endocrinology. And among a certain subset of the endocrine patients he saw in his adolescent medicine clinic, he discovered something unsettling: "Sometimes there was a kind of conspiracy between the doctors who cared for them years before, and the parents," says Spack. A conspiracy "to not tell what had happened to them."

As many as one in 100 babies are born with genitals, chromosomes, hormonal systems — or some combination of these — that don't fit the standard parameters for male or female. Today, treating kids with what are now known as "disorders of sexual differentiation," or DSD, is one of GeMS's specialties, and Spack routinely helps families decide whether to raise their child as male or female and weighs in on whether and when surgery is required. In making this decision, they consider "diagnosis, genital appearance, surgical options, need for lifelong replacement therapy, potential for fertility, views of the family, and, sometimes, circumstances relating to cultural practices," Spack and his colleagues wrote in a landmark 2006 consensus statement in the journal Pediatrics on the management of intersex disorders.

But these guidelines were developed partly in response to increasingly vocal activists like Intersex Society of North America founder Cheryl Chase, who as an infant had her clitoris surgically shortened. Before people like Chase began to speak up in the early '90s, parents of these kids — even babies who were unambiguously sexed except for a particularly small penis or a large clitoris — were often pressured into sex reassignment and early genital surgery to make their infants appear more "normal." In some cases, the condition was treated as an emergency, and parents were not even consulted until after the surgery was over.

By the time Spack began his adolescent medicine practice, it was not uncommon for him to meet teenagers who had never been told that these surgeries had been performed on them as infants.

"In the days before the computer, you could seal medical records," Spack recalls. "You could literally control that information." But one of the reasons Spack was drawn to working with adolescents in the first place was that he liked to have independent relationships with his patients — developing trust with teenagers gave him a "direct line" to them that bypassed their parents — and he felt strongly that these kids needed to know what had happened to them.

"This was in an era when chromosomes were so important," says Spack. "If somebody's chromosomes didn't match the way they were raised, many parents didn't want the kids to know." For instance, he saw young women who "never knew that they were born with a male set of chromosomes. There were a number of circumstances where I would tell them and make them understand how meaningless their chromosomes were." The parents feared that the information would "do something to their gender identity," but Spack consistently found that it didn't.

While he worked with young intersex patients during the day, at night he volunteered one evening a month on a medical outreach van with the organization Bridge Over Troubled Waters. Here he encountered members of the LGBTQ community professionally for the first time. The "Bridge kids" were hustling for sex, some of them in drag bars, dressed as women.

Spack wasn't sure whether they were gay, transsexual, or just runaways doing what they needed to do to get by.

"I was naïve," he says now. "I thought that all the kids I saw in the streets were runaways. And I had been told about the rules about runaways, like that they could be housed for three days without calling the parents. But one of the street workers said, 'They call them runaways. These kids are throwaways. Our goals aren't to get the kid home. Home is not a safe place.' "

His 10 years working with Bridge kids left a lifelong impression on Spack, who is acutely aware of the "kind of ideal world" that GeMS selects for when it screens its patients. "Both parents have to agree, and you have therapists who have weighed in, good people you trust."

This, he knows, is sadly uncommon. Even transgender children with one supportive parent, he says, are "one parent away from being a Bridge kid."